Advances in prior art diode lasers have generally concentrated on improving the performance of individual laser emitters. Gain guided lasers, and lasers incorporating lateral real refractive index waveguiding are discussed in W. Streifer, R. D. Burnham, D. R. Scifres, "Current Status of (GaAl)As Diode Lasers," SPIE Vol. 269 Integrated Optics, pages 1-7, February, 1981. These lasers have lower threshold currents, increased efficiency and better beam characteristics than the first diode lasers.
The buried-heterostructure diode injection laser, disclosed in U.S. Pat. No. 4,033,796 to Burnham et al., is formed by etching a groove or channel into a substrate. An active layer has a bowl-shaped central portion that is completely surrounded by light guiding, current confining layers. Light waves produced by this laser are guided in the central portion of the active layer, making lowest order transverse modes possible in CW room temperature operation.
It is desirable to fabricate diode laser bars with a linear array of laser beam emitting segments along the width of a bar face. Such a laser bar may be butted against Nd:YAG media to provide optical pumping of Nd:YAG lasers. A laser bar could be constructed by combining individual laser emitters. However, the lateral packing density, i.e. the percentage ratio of the width of the light emitting regions to the total width of the diod laser bar would be small. For example, a channeled substrate laser emits a five micron wide beam from a 250-micron-wide crystal face, making the packing density only two percent.
Greater power output capability is required for diode laser bars to be useful in such high power applications as Nd:YAG laser pumping and the like. Factors which improve the output power include phase locking of the array, increasing the total number of light-emitting regions, and widening the emitting width as with a wide stripe laser. However, based on past designs the width of the emitting regions had to be limited in order to suppress lateral superradiance. Such suppression was achieved via light absorption outside the pumped stripe width. Lateral superradiance, which is amplified spontaneous emission in the lateral direction, consumes power which might otherwise go into useful light amplification in the longitudinal direction. Unfortunately, as the packing density increases, and as the width of a laser bar increases in order to increase the width of or total number of light-emitting regions, superradiance increases to a point where it is a serious power drain.
Wide oxide stripe laser bars have been fabricated which emit a multitude of gain-guided beams from their emitting face. A plurality of periodically spaced parallel stripes of oxide material, proton bombarded regions, or other insulating stripes are deposited on the top layer of the laser diode and covered with a metallization layer. Those regions of the active layer beneath metal contact areas are pumped, while those regions beneath the insulating stripes are absorbing. Typically, the pumped regions might be 60 microns wide, while the laser chip is 250 .mu.m wide. Thus, the packing density for this chip is about 25%. There is sufficient absorption on either side of the gain region to suppress superradiance. Furthermore, the typical cavity length is 250 .mu.m so that the gain in the longitudinal direction far exceeds the gain in the transverse (60 .mu.m) direction which suppresses superradiance.
In an article entitled "High-Power Individually Addressable Monolithic Array of Constricted Double Heterojunction Large-Optical-Cavity Lasers" by D. Boetz et al. in Applied Physics Letters, December, 1982, pp. 1040-1042, a linear array of ten emitters is disclosed. The array features a convex lens-shaped active layer and a concave lens-shaped guide layer with oxide stripes for current confinement. The linear packing density is less than 10%.
A packing density close to 100 percent, i.e. laser emission across nearly the entire width of the bar, is preferred. The oxide insulation could be eliminated and the entire active region could then be pumped. However, as the emitting segment along a bar becomes wider than about 500 microns, for a 250 microns long laser (distance between cleaved mirrors is 250 microns) lateral superradiance, i.e. stimulated emission in the direction parallel to the cleaved end mirrors, and the p-n junction, i.e., perpendicular to the line between the mirrors, may occur. Oxide stripe lasers having a packing density less than 50 percent avoid this problem because of their large absorbing regions, but as the packing density increases, the amount of coupling between emitting segments increases, and light becomes amplified in each gain section by more than it is attenuated in each absorbing section. So wide oxide stripe lasers with a packing density greater than 50 percent have undesirable lateral superradiance.
Phased locking or wide pumped region constructions have greatly in-creased the power output capability of diode lasers. Insulating stripe laser bars have been phase-locked for packing densities of up to 40 percent. For example, a 1 mm wide laser bar constructed with 40 insulating stripes on 10 m spacings had a phase-locked emitting region 400 microns wide. However, when the phase-locked emitting region was extended to 600 microns wide, lateral superradiance resulted.
In. U.S. Pat. No. 4,594,718, Scifres et al. disclose a semiconductor laser array which comprises a substrate, a cladding layer, active layer(s), cladding layer and contact layer. An optical cavity is provided in the longitudinal direction of the active region between end facets and a plurality of laser beams are emitted from a plurality of waveguides. The laser has current confinement regions for pumping the active region along each waveguide. The laser array substrate has a cross-shaped channel or mesa configuration provided with a contiguous central gain-guiding region that extends transversely through the laser array with a plurality of narrow index guiding regions or waveguides extending from the central region to either end facet. Each waveguide supports the propagation of only a single transverse mode. The configuration allows coupling of light in the gain-guided sections for phase-locked operation.
In the laser array, only the channel portions form waveguides that allow light to propagate under lasing conditions, because the cross-shaped configuration introduces steps between the mesa portions of the index guided regions and the gain-guided region, which steps cause any longitudinally propagating light in the mesa portions to be scattered or deflected out of the active region. Similarly, an alternate embodiment introduces steps between channel portions of the index-guided regions and the gain-guided region so that only mesa portions allow light propagation under lasing conditions. Further, the gain-guided region functions very much like the oxide stripe laser bars mentioned above. When the pumped portion of the gain-guiding region is less than 50 percent of the total gain-guiding region, lateral superradiance is avoided because of the large absorbing regions between the pumped portions. Were the pumped portions to be increased above 50 percent, undesirable lateral superradiance in the gain-guiding region would result. Accordingly, only those portions of the gain-guiding region corresponding to channel portions (alternatively mesa portions) are pumped. (The channel portions themselves avoid superradiance since they only support a single transverse mode, and are strongly indexguiding so as to prevent optical coupling between waveguides.) For these reasons, the laser array of the '718 patent has a maximum packing density of about 45-50 percent.
An object of the invention is to provide a diode laser bar having a packing density up to near 100 percent that does not have lateral superradiance, especially for laser bars with multimode waveguides.